1. Field of the Invention
The present invention relates to an audio scrambling system for scrambling audio signals on the transmission side for enhanced security, and descrambling the scrambled audio signal on the receiving side. This audio scrambling system may be used for sending and receiving audio signals using wired or wireless communications, or for recording and reproducing audio signals using a storage medium.
2. Description of the Prior Art
One audio scrambling system according to the prior art is described in U.S. Pat. No. 5,091,941 (filed Oct. 31, 1990, Ser. No. 607,988). This audio scrambling system scrambles the signal by randomizing only the sign bit of each sample in a digital audio signal. The effect of this is to prevent the volume of the audio after scrambling from becoming abnormally high. However, there is only one confidential bit per sample, and the scrambled audio signal can be decoded by unauthorized parties with relative ease.
Another prior-art audio scrambling system is described in Japanese patent laid-open publication number 63-87037. With this audio scrambling system, the transmission side extracts the amplitude data of the audio signal, and scrambles the data only when the amplitude data is less than a predetermined level. The receiving side receives the amplitude data and the scrambled audio, and only descrambles the signal when the amplitude data is less than a predetermined level. This method also prevents the volume of the audio after scrambling from becoming abnormally high. However, because these conventional devices must also transmit amplitude data in addition to the audio data, a separate transmission path is required.
An encryption device used in communications systems which use a specific data pattern as a control code is described in Japanese patent laid-open publication number 4-68387. With this encryption device, a conversion table is compiled and used for encryption. With this conversion table, the control code itself is converted to the same control code, i.e., converted but without any change, and non-control codes are converted to codes other than the control code.
To prevent control errors in communications systems, however, it is necessary to prevent the control code from being converted to another code, and to prevent other codes from being converted to the control code. Control codes of this type are called "reserved codes." The process whereby the reserved code is not converted to another code, and non-reserved codes are not converted to the reserved code, is known as "reserved code management." Conventional encryption devices combine the encryption process with reserved code management, and can thereby prevent control errors in communications systems.
With this conventional encryption system, however, time is required to pre-compile the conversion table, and a storage means is required to store the conversion table, thus increasing both the scale of the device and the required processing time. In addition, the encryption process can only be executed based on a conversion table.